The hitec servo you posted is really slow for 1/10 steering servo. You need something closer to the .10 sec for your transition time.
If you want to stick with Hitec look for 6975 (144oz, .10sec), or the ultimate steering servo Hitec 7940 (180oz, .07sec At 6.0V) But you can also run it at 7.4V for even better performance.
Shawn.